This invention relates to spread spectrum communications, and more particularly to a frequency hopping, code division multiple access, microcellular or cellular communications system, in which the maximum capacity per sector can approach or exceed the total bandwidth of the system divided by the bandwidth occupied by the frequency hopping signal during any dwell. This ratio is called the processing gain.
The frequency spectrum is extremely congested. To ensure that the worldwide need for increased communication services is met, more spectrum must be found. In an attempt to meet these needs, the Federal Communications Commission (FCC) has set aside the frequency bands of 1850–1990 MHz, 2110–2150 MHz, and 2160 2200 MHz, for emerging technology services. The 1850–1990 MHz band has been set aside for Personal Communication Services (PCS). Additionally, the FCC has allocated spectrum under part 15 of the FCC rules, for spread spectrum use.
PCS is expected to become, during the 1990s, a business of significantly more than $100 billion, annually. PCS is a One Phone concept, in which a single phone can be used in the home in lieu of a cordless phone; in the street with a wireless local loop; in the vehicle with a cellular type system; and, in the office with a wireless private branch exchange (PBX). The One Phone will provide wired-line quality voice and integrated service digital network (ISDN) data rates, with wireless convenience. Used with an intelligent network, a call placed to a user will reach the user, no matter where the user is located.
The PCS frequency band is also inhabited by licensed, fixed service, microwave users. These microwave users transmit data point-to-point using towers approximately 150–200 feet high, separated by about 10–20 miles. Their systems employ one watt power amplifiers and four degree beamwidth antennas.
In earlier experiments, a direct-sequence spread spectrum CDMA system was demonstrated to share, i.e., overlay, the spectrum with the microwave users without causing significant attenuation, by limiting the capacity of the CDMA system. The co-sharing proposal centered about using a 48 MHz bandwidth for the CDMA system. Thus, the power reaching the fixed microwave receiver from a PCS user was attenuated by 7 dB, i.e., 4.8. Additional attenuation resulted because the PCS user is not within the narrow beamwidth of the fixed service microwave user's antenna. The FCC's ruling limits the bandwidth to 15–20 MHz full duplex. Thus, a 15–20 MHz band-can be allocated for transmission and a second 15–20 MHz band can be allocated for reception. This ruling reduces the direct-sequence spread spectrum capacity by about 4 to 5 dB. These results are shown in Table I for experimental data taken in the suburbs of Orlando and Houston, and in densely urban New York, using a 48 MHz bandwidth. The results presented in this table for the 15 MHz bandwidth were obtained by multiplying the first column by 15/48. The reason that more users could overlay in New York City is that the tall buildings blocked the line-of-sight path between a PCS user and microwave user. Hence, any PCS signal reaching the microwave antenna is severely attenuated.
TABLE 1Number of Users/Base StationExperimental ResultsCalculated Results(48 MHz Bandwidth)(15 MHz Bandwidth)Houston4614Orlando3410New York City538 168 
If no overlay were needed, i.e., all microwave users were removed from the frequency band, then the maximum capacity, C, of a PCS microcellular system could beC˜fc/fb.  (1)where fc is the chip rate of the direct sequence (DS) spread spectrum system and fb is the data rate. Using a data rate of 32 kilobits per second (kb/s) to ensure toll quality voice and a bandwidth of 48 MHz, each cell could service up to 538 simultaneous calls while a 15 MHz bandwidth system could handle up to 168 simultaneous calls.
In order to eventually achieve the much higher capacity possible in the non-overlay mode, the FCC has given the microwave users 3–10 years to leave the PCS band and move to a higher, frequency band. During the interim period, overlay will enable the PCS business to develop in a reasonable fashion and provide or a smooth transition.
Frequency hopping (FH) CDMA is an alternate approach which allows sharing the band with the microwave users by hopping over them, i.e., by excluding frequencies used by the microwave users. A frequency hopping signal typically is a quadrature phase shift keyed (QPSK) or binary frequency shift keyed (BFSK) signal which changes frequency at regular time intervals, called the hop duration. If the bandwidth occupied by the frequency hopping signal during any dwell were fb and the total bandwidth of the system were B, then the system capacity, C, ideally becomesC=B/fb  (2)Comparing Equations (1) and (2), the FH/CDMA and the DS/CDMA systems have approximately the same capacity if fc=B. Typically, however, the capacity of a FH/CDMA system is limited to a much lower value than given by Equation (2). The reasons for this are:    1. If N frequency-hopping users change frequency at random times and with a pseudo random sequence, then the probability of two users landing on the same frequency at the same time, thus causing a collision and producing errors, increases rapidly with N, thereby limiting N to about 10% of capacity.    2. If users in one cell used all C frequency “bins”, then the users in adjacent cells would have to use the same frequencies, thereby interfering with one another. To avoid such interference, frequency reuse could be employed, thereby limiting the number of users to about 1/7, or 15% capacity.
Accordingly, there exists a need for a system which avoids the limitations disclosed above.